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Monday, June 9, 2014

Real Life weight calculation Question and Answer

A reader said "I have a  32' Class A MH and am about to pull the trigger on tires. I have been so concerned about making sure I get the 'right' tires, meaning, ones that have the right load range and capacity.

So just today I FINALLY found a place to weigh the MH and here's what I have:
     Actual Weight          Sticker Info
Front Axle      4,960          6,000
Rear Axle    10,660          11,000
Total            15,620          17,000

I was concerned that the 235/85R16-E 10 ply load range of 3,042 was too close to the limit so instead of I was thinking of getting a G 14 ply rated tire.

But now that I see the actual weight, should I stick with the E - 10 ply range?

Here's additional info: The RV had 3/4 tank of gas, full tank of clean water, very little in gray water and me (175 lbs)

And is this how you figure "Reserve Capacity"?

Tire rating for 235/85R16 Load Range E = 3,042       
So is the reserve load range capacity minus actual weight?       
Front Capacity           3,042 x 2       6,084
Rear Capacity            3,042 x 4      12,168
Front Axle Weight         4,960
Rear Axle Weight         10,660
Front Reserve             6,084 - 4,960       1,124 - I can add this much weight to the front
Rear Reserve            12,168 - 10,660     1,508 - and this much to the back

Compare to Hercules H-901 Load Range G = 3756 (3415 Dual..why?)
Front Capacity     3,756 x 2 = 7,512
Rear Capacity      3,415 x 4  = 13,660
Front Reserve       7,512-4,960 = 2,552 *Much better compared to 1,124 lbs
Rear Reserve        13,660-10,660 = 3,000 *Much better compared to 1,508 lbs

So, that I have actually put it all down on paper it's kind of a 'duh' moment, but here's the price difference:

6 - Cooper A/T3 - $1,159 Out the door
6 - Hercules H-901 - $1,532 OTD
Difference $373

Worth it?"

I offered the following reply
"Some comments and observations.

It helps if you use the complete tire type/size nomenclature.
I believe you have LT235/85R16 LR-E   the LT is Type. Some readers may have a trailer and have ST type tires and there are full metric tires with no leading letters. Each type has different load capabilities and ST are not intended for passenger carrying vehicles.

Second you failed to consider the correct load capacity in the dual position as your rear tires are. Tires in dual application ALWAYS have a lower rating than when in single as one of the pair is almost ALWAYS carrying more than 1/2 the load of the pair.
When you either read the tire sidewall or look at Load/Inflation charts you will see a different load capability for Single (as in your front tire) or Dual (two tires side to side as in your rear.

The load capacity for an LT235/85R16 LR-E in Dual application is probably 2,778#.  Note you should always consult the Load/Inflation tables or molded on the sidewall from the tire manufacturer of the tires you are considering as not all tires have the same load numbers. Some are slightly different for a variety of reasons.

So back to your questions.

"Reserve Capacity" would be the (max tire capability) - (the actual load) on a tire.
BUT you cannot simply assume the load on an axle is split 50/50 side to side.
Some owners find significant variation so untll you learn the individual tire loads I suggest you assume that one side is carrying 55% of the axle load.

Your heavy side Front could have 55% OF 4,960# OR 2,728#
Rear 55% of 10,660# or 5963#

Your front Reserve is 3,042#- 2,728# or 314# per tire

Your rear would be  1/2 of 5,963# or 2,982#
so the rear reserve is 2,778# - 2,982#  or a negative reserve or potential tire overload of 204# per tire.

With this information it is important that you first confirm the side to side load balance

Here is a worksheet

After confirming the real side to side split you need to re-calculate your reserve load and if needed unload or shift the load of stuff in the RV or buy the tires with more load capability BUT you should never exceed the individual axle ratings  GAWR or Vehicle rating GVWR shown on your certification label.

Finally the selection of tire brands. Do both brands offer the same warranty? Can you find dealers for both brands in the part of the country you normally travel? If you get a puncture and need a replacement tire it could end up costing more than a couple hundred dollars extra if you need a tire rush shipped in to match your current tires.

Hope this helps

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1 comment:

  1. Almost all tire manufacturers have similar worksheets to show proper weighing of an RV. Due to factors mentioned in this article - unbalanced load - more weight on one side than the other - it is extremely important to weigh an RV properly. But not only for tires but all other components of the suspension - springs - shocks - axles - brakes - etc.. Every component has a weight limit. Axle weighing without doing side to side weighing does not tell all the facts. Facts are important for safety and handling. All RVers should find a way to properly weigh. Weight can kill you.


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